Few-layer large Ti3C2Tx sheets exfoliated by NaHF2 and applied to the sodium-ion battery

Abstract

Since the original discovery, MXene can be synthesized by wet-chemical etching from the MAX phase. To avoid the shortcomings of the previous etchant, NaHF₂ aqueous solution was used for the selective removal of Al from Ti₃AlC₂, while intercalating Na⁺ between layers. The d-spacing of the resulting accordion-like Ti₃C₂T_x is up to 12.35 Å; the by-product Na₃AlF₆ can be removed by H₂SO₄, and the product yield can reach about 85%. After a short time of centrifugation and sonication, few-layer (10–20 layers) Ti₃C₂T_x sheets in a large lateral size (3–4 μm) can be prepared, with high conductivity up to 2.3 × 10⁵ S m⁻¹. Ti₃C₂T_x can be stably dispersed in water and organic solvents for at least 96 h, and has excellent anti-thermal-oxidation ability. As the anode for sodium ion batteries, the Ti₃C₂T_x electrode delivers a high reversible capacity of 70 mA h g⁻¹ after 900 cycles at a current density of 1 A g⁻¹ with an extremely low-capacity decay rate. From 900 to 1000 cycles, the capacity can even be increased to 130 mA h g⁻¹, and the Coulomb efficiency is close to 100%, showing that MXene can provide a stable structure for the Na⁺ cycle.